1. Field of the Invention
The present invention relates to a semiconductor light emitting element, especially, relates to a tunable wavelength laser light source enabling laser oscillation at desired wavelengths, and a super luminescent diode enabling the expansion of the half width of the emission spectrum.
2. Prior Art
With the widespread use of multimedia, several kinds of methods for increasing the capacity of optical communication systems have been proposed.
One example of the methods is a wavelength division multiplexing (WDM) communication using erbium doped fiber amplifiers (EDFAs) or Raman amplifiers. The WDM communication system can increase the capacity of the communication system drastically without increasing the number of optical fibers installed, because of the fact that the system propagates information by using a plurality of lights having different wavelengths.
In the above mentioned WDM communication system, a signal light source comprises a plurality of distributed feedback laser diodes (DFB-LDs) where each DFB-LD oscillates at one of several specific wavelengths that are different from each other, corresponding to each communication channel. In order to improve the reliability of the communication system, however, it is necessary to install spare DFB-LDs for all the DFB-LDs for backup purposes, resulting in higher cost.
Therefore, there has been a need for a tunable wavelength laser enabling laser oscillation at desired wavelengths in order to back up all the channels with less number of the tunable wavelength laser than the number of the DFB-LDs.
The applicant has proposed a tunable wavelength laser available for the above mentioned purpose (see, for example, Japanese published unexamined application No. H06-5980).
The tunable wavelength laser according to the above proposal has two regions aligned on the resonant axis, which are a light emitting region and a wavelength controlling region having a heating means. The oscillation wavelength is controlled by heating the wavelength controlling region from the heating means.
The tunable wavelength laser according to the above proposal, however, is a monolithic laser, which has a phase controlling region and a distributed Bragg reflector (DBR) region having a grating as the wavelength controlling region. Therefore, the wavelength change is restricted by the refractive index change, resulting in a maximum wavelength change of about 15 nm in practical use. Because of the fact that the tunable wavelength laser comprises the coupling of an active waveguide and a passive waveguide, there are some problems such that the number of crystal growth processes is large, the manufacturing cost is high, and the yield is poor.
A tunable wavelength light source using an external cavity laser device has been widely used mainly as a light source for measurement purposes (see, for example, Japanese published unexamined application No. 1107-335965).
The tunable wavelength light source according to the above proposal has a laser device having two regions aligned on the resonant axis, which are an active region and a phase controlling region, and a grating which can control the diffraction wavelength. The oscillation wavelength is controlled by controlling the reflection wavelength of the grating and the refractive index of the phase controlling region.
The tunable wavelength light source according to the above proposal, however, also has some problems such that the crystal growth of the active waveguide and the passive waveguide needs to be performed separately, ensuring high enough resistance between electrodes is difficult, the line width of the emission spectrum broadens, and the reliability of the control of the refractive index of the passive waveguide is not established.
That is, because of the fact that the phase controlling region comprises a semiconductor having a larger band gap energy than that of the active region, the crystal growth of the active waveguide and the passive waveguide needs to be performed separately, resulting in the increase in the number of crystal growth processes.
Since enhancing the resistance between electrodes is difficult, interference between the current injected into the phase controlling region and the current injected into the active region is inevitable.
Due to the fluctuation of the refractive index in the phase controlling region, resulting from the shot noise caused by free carriers, the expansion of the line width of the emission spectrum is inevitable. The tunable wavelength light source according to the above proposal also has a problem that increasing the output is difficult because of the fact that the light is absorbed due to the free carrier absorption.
As for the tunable wavelength light source according to the above proposal, the refractive index of the phase controlling region is changed due to the plasma effect caused by the injection of the current into the phase controlling region. It has been known that the device easily deteriorates when the condition of high carrier density continues for long time, therefore ensuring the reliability of the device is difficult.
The present invention to solve the above described problems, therefore, aims to provide a semiconductor light emitting element which can control the phase when applied to an external cavity laser, and can broaden the width of emission spectrum when applied to a super luminescent diode.